A toner and a carrier included in a developer are previously mixed at a specified concentration, and are put in a developing process unit having an agitation and mixing mechanism. Here, the toner and the carrier are mixed by the agitation and mixing mechanism, and by this, the toner is friction charged and is attached to a latent image of a photoconductor, and it is subjected to development.
It is known that the developer deteriorates due to long-term use, and its fluidity is reduced. Since the fluidity is reduced, the agitation state is changed between at the initial stage and after long-term use, and it becomes difficult to keep the charging performance of the initial toner.
When such a developer is used over a long period of time, although a new toner is supplied according to the consumption of toner, the developer deteriorates due to the deterioration of the carrier by being spent, the separation of an additive from the surface of the toner, a degraded toner that is not developed but remains on a photoconductor and has poor charging performance, and various disadvantages occur when the developer is near the end of the life.
As a method of keeping the developer characteristic for a long period of time, there is disclosed a method in which a small amount of carrier is mixed with a supplementary toner and is supplemented whenever necessary, and the performance of the carrier deteriorated by being spent is supplemented, or a method in which a developing bias is variably controlled according to the life time, and the developing operation of the toner whose charging performance is reduced is supplemented. However, in the above methods, a disadvantage in an image due to a reduction in fluidity of the developer, which occurs in the case of long-term use, has not been resolved.
In order to resolve the disadvantage of the image due to the change of the fluidity in the life of the developer, for example, JP-A-6-167886 discloses a method in which a fluidity sensor is installed in a developing unit, the driving force of agitation means is increased by a detection signal from the sensor when the fluidity of the developer is lowered, and the fluidity of the developer is recovered. Although this method is effective in the recovery of the fluidity of the developer, since it is necessary to install the fluidity sensor in the developing unit and to provide means for detecting the signal from the sensor, the structure of the developing unit becomes complicated and the cost is high.
Besides, JP-A-6-59571 discloses a method in which in a developing apparatus, the fluidity of a toner is measured by detecting the amount of the toner transported from a toner hopper to a developing unit, and the rotation speed or rotation time of a rotator to supply the toner from the toner hopper to the developing unit is controlled. In this method, although it is possible to detect the fluidity of only the toner, it is difficult to detect the fluidity of the developer in which the toner and carrier are mixed.